public function set_member_types( $member_types, $append = false ) {
		// Unset invalid member types.
		$types = array();
		foreach ( $member_types as $member_type ) {
			// 'null' is a special case - it represents users without a type.
			if ( 'null' === $member_type || bp_get_member_type_object( $member_type ) ) {
				$types[] = $member_type;
			}
		}

		// When `$append` is false, delete all existing types before adding new ones.
		if ( ! $append ) {
			bp_xprofile_delete_meta( $this->id, 'field', 'member_type' );

			/*
			 * We interpret an empty array as disassociating the field from all types. This is
			 * represented internally with the '_none' flag.
			 */
			if ( empty( $types ) ) {
				bp_xprofile_add_meta( $this->id, 'field', 'member_type', '_none' );
			}
		}

		/*
		 * Unrestricted fields are represented in the database as having no 'member_type'.
		 * We detect whether a field is being set to unrestricted by checking whether the
		 * list of types passed to the method is the same as the list of registered types,
		 * plus the 'null' pseudo-type.
		 */
		$_rtypes  = bp_get_member_types();
		$rtypes   = array_values( $_rtypes );
		$rtypes[] = 'null';

		sort( $types );
		sort( $rtypes );

		// Only save if this is a restricted field.
		if ( $types !== $rtypes ) {
			// Save new types.
			foreach ( $types as $type ) {
				bp_xprofile_add_meta( $this->id, 'field', 'member_type', $type );
			}
		}

		// Reset internal cache of member types.
		$this->member_types = null;

		/**
		 * Fires after a field's member types have been updated.
		 *
		 * @since 2.4.0
		 *
		 * @param BP_XProfile_Field $this Field object.
		 */
		do_action( 'bp_xprofile_field_set_member_type', $this );

		// Refetch fresh items from the database.
		return $this->get_member_types();
	}